Method and system for separating a component from a multi-component gas

ABSTRACT

A method and apparatus for separating a component from a multi-component feed gas stream has a flow conduit ( 14 ) having a semi-permeable section ( 15 ) that permeates the component to be separated from feed gas stream ( 12 ). A sweep gas is provided at a first velocity on the permeate sides of flow conduit ( 14 ) and the velocity of sweep gas ( 13 ) is accelerated so that the velocity of sweep gas ( 13 ) along at least a portion of the permeate side of flow conduit ( 14 ) is greater than the first velocity. The mixture of permeate and sweep gas ( 13 ) is then decelerated by diffuser ( 20 ), thereby recovering as pressure a portion of the energy of feed gas stream ( 12 ).

FIELD OF THE INVENTION

The present invention relates generally to a semi-permeable,gas-separation system for separating one or more components from amulti-component gas.

BACKGROUND OF THE INVENTION

Natural gas is an important fuel gas and it is used extensively as abasic raw material in the petrochemical and other chemical processindustries. The composition of natural gas varies widely from field tofield. Many natural gas reservoirs contain relatively low percentages ofhydrocarbons (less than 40%, for example) and high percentages of acidgases, principally carbon dioxide, but also hydrogen sulfide, carbonylsulfide, carbon disulfide and various mercaptans. Removal of acid gasesfrom natural gas produced in remote locations is desirable to provideconditioned or sweet, dry natural gas either for delivery to a pipeline,natural gas liquids recovery, helium recovery, conversion to liquefiednatural gas (LNG), or for subsequent nitrogen rejection. H₂S is removedbecause it is toxic in minute amounts and it is corrosive in thepresence of water through the formation of hydrosulfurous acid. Uponcombustion, H₂S forms sulfur dioxide, a toxic and corrosive compound.CO₂ is also corrosive in the presence of water, and it can form dry ice,hydrates and can cause freeze-up problems in pipelines and in cryogenicequipment often used in processing natural gas. Also, by notcontributing to the heating value, CO₂ merely adds to the cost of gastransmission.

An important aspect of any natural gas treating process is economics.Natural gas is typically treated in high volumes, making even slightdifferences in capital and operating costs of the treating unitsignificant factors in the selection of process technology. Some naturalgas resources are now uneconomical to produce because of processingcosts. There is a continuing need for improved natural gas treatingprocesses that have high reliability and represent simplicity ofoperation.

A number of processes for the recovery or removal of carbon dioxide fromgas steams have been proposed and practiced on a commercial scale. Theprocesses vary widely, but generally involve some form of solventabsorption, adsorption on a porous adsorbent, distillation, or diffusionthrough a semipermeable membrane.

Membranes are thin barriers that allow preferential passage of certaincomponents of a multi-component gas mixture. Most membranes can beseparated into two types; porous and nonporous. Porous membranesseparate gases based on molecular size and/or differential adsorption bysmall pores in the membrane. Gas separation membranes used in naturalgas applications are often nonporous or asymmetric and separate gasesbased on solubility and diffusivity. These membranes typically have amicroporous layer, one side of which is covered with a thin, nonporous“skin” or surface layer. The separation of the gas mixtures through anasymmetric membrane occurs in its skin, while the microporous substrategives the membrane mechanical strength.

In a typical membrane separation process, a gas is introduced into thefeed side of a module tat is separated into two compartments by thepermeable membrane. The gas stream flows along the surface of tomembrane and the more permeable components of the gas pass through themembrane barrier at a higher rate than those components of lowerpermeability. After contacting the membrane, the depleted feed gasresidue stream, to retentate, is removed from contact wit the membraneby a suitable outlet on the feed compartment side of the module. The gason the other side of the membrane, the permeate, is removed from contactwith the membranes the permeate side, through a separate outlet. Thepermeate stream from the membrane may be referred to as being “enriched”in the readily permeable components relative to the concentration of thereadily permeable components in the retentate stream. The retentate mayalso be referred to as being “depleted” of the readily permeablecomponents. While the permeate stream can represent the desired product,in most natural gas permeation processes the desired product is theretentate stream, and the permeate stream comprises contaminants such asCO₂ or other acid gases.

The efficiency of a membrane depends on many factors including thepressure differential being maintained across the membrane, whereby thepermeable fluid component(s) permeate to the permeate side of themembrane under a partial pressure gradient. In order to maintain thepartial pressure differential across the membrane, a sweep fluid isoften used to help remove the permeating fluid. The lower the partialpressure of the permeate, the better the separation. This is especiallyimportant in applications where only small amounts of fluid are to beseparated from the fluid mixture. However, many uses for the permeaterequire further pressurization of the permeate. Low permeate partialpressure is desired for efficient membrane application, but highpermeate pressure is desired to reduce compression costs.

While membrane systems that use sweep fluids ha been effective inimproving the efficiency of membrane separation of fluid, there is acontinuing need for improving to efficiency of membrane separationprocesses.

This invention provides a method and system for separating at least onegaseous or vaporous component from a multi-component gas stream. A flowconduit is provided having a semi-permeable section adapted toselectively permeate the at least one gaseous component to be separatedin the presence of the multi-component gas flowing along one side of thesemi-permeable section. The multi-component gas is passed along the feedside of the flow conduit and a sweep gas, having a first velocity, isprovided for passage along the permeate side of the flow conduit, thesweep gas being suitable for sweeping the component gas that permeatesthrough the permeable section of the conduit away from the permeate sideof the flow conduit, thereby producing a gas mixture comprising thesweep gas and the component gas. The velocity of the sweep gas isaccelerated so that the velocity of the sweep gas along at least aportion of the permeate side of the flow conduit is greater than tofirst velocity of the sweep gas. The gas mixture is then decelerated bymeans of a defuser, thereby recovering as pressure a portion of theenergy of the gas mixture.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention and its advantages will be better understood byreferring to the following detailed description and the attacheddrawings, in which like reference numerals are used to indicate likeparts in various views.

FIG. 1 is a sectional, schematic view of one embodiment of the presentinvention showing a feed gas conduit with a portion of the conduit'sstructure being semi-permeable and a nozzle conduit surrounding aportion of the feed conduit for collecting permeate and for passingsweep gas across the semi-permeable structure at subsonic velocity.

FIG. 2 is a cross-sectional view of the embodiment shown in FIG. 1 takenalong lines 2-2.

FIG. 3 is a sectional, schematic view of a second embodiment of thepresent invention similar to FIG. 1 except that the nozzle conduitsurrounding the semi-permeable structure provides for supersonicvelocity of the sweep gas across the semi-permeable structure.

FIG. 4 is a sectional, schematic view of a third embodiment of thepresent invention showing a nozzle conduit on the inside of a membraneconduit, the nozzle conduit collecting permeate and providing passage ofsweep fluid at subsonic velocity.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an apparatus and method for separatingone or more components from a multi-component gas using a separationsystem having a feed side and a permeate side separated by asemi-permeable structure. The separation system uses a sweep gas tofacilitate removal of permeate from the permeate side of the separationsystem. This present invention increases the velocity of sweep gas onthe permeate side of the semi-permeable structure by reducing thecross-sectional area of sweep gas flow, thereby increasing the velocityof the sweep gas and reducing the static pressure of the permeate on thepermeate side of the structure. The reduction in static pressure isachieved in one embodiment by using a converging nozzle for subsonicflow velocities and in another embodiment by using aconverging-diverging nozzle for supersonic flow velocities.

The terms used in this description are defined as follows:

“Effuser” means a flow channel having a convergent section downstream offlowing section which functions as an aerodynamic expander. An effusermay have a converging volume or a converging and then diverging volume.

“Supersonic effuser” means a flow channel having a convergent subsonicsection upstream of a divergent supersonic section with an interveningsonic throat which functions as an aerodynamic expander.

“Diffuser” means a flow channel having downstream divergent sectionwhich functions as an aerodynamic compressor. A diffuser may have adiverging volume or a converging and then diverging volume.

“Supersonic diffuser” means a flow channel having a convergent supersonic section upstream of a divergent subsonic section with anintervening sonic throat which functions as an aerodynamic compressor.

“Throat” means a reduced area in a flow channel, as in an effuser ordiffuser.

“Natural gas” refers to a multi-component gas obtained from a crude oilwell (associated gas) or from a subterranean gas-bearing formation(non-associated gas). The composition and pressure of natural gas canvary significantly. A typical natural gas stream contains methane (C₁)as a significant component. The natural gas will also typically containethane (C₂), higher molecular weight hydrocarbons, one or more acidgases (such as carbon dioxide, hydrogen sulfide, carbonyl sulfide,carbon disulfide, and mercaptans), and minor amounts of contaminantssuch as water, nitrogen, iron sulfide, wax, and crude oil.

The present invention is particularly suitable for treatment of naturalgas streams containing one or more contaminants such as carbon dioxide,hydrogen sulfide, and water vapor. However, the invention is not limitedto treatment of natural gas. The inventive device and method can be usedto separate multi-component gas, in which a semi-permeable membrane isused to separate one or more components of the multi-component gas.

FIG. 1 schematically illustrates one embodiment of a fluid separationdevice 10 of the present invention. A multi-component feed gas streamenters fluid separation device 10 through flow conduit 14. Arrow 12shows the flow direction of the feed gas. A portion of conduit 14 has asemi-permeable structure 15 through which one or more components of amulti-component fluid stream 12 can selectively permeate therethrough.Arrows 16 show the direction of the permeate through conduit structure15. A sweep fluid is passed through flow conduit 17 in the direction ofarrow 13. Flow conduit 17 has a converging section 18 which causes thesweep gas stream to accelerate. Downstream of the converging section 18,the cross-sectional area of the sweep conduit 17 can remain constant,progressively increase or, as shown in FIG. 1, progressively decrease.By conduit 17 progressively decreasing along the length of thesemi-permeable structure 15 the velocity of the sweep gas can bemaintained as the permeate that mixes with the sweep gas increases themass flow of the mixture along the length of the structure 15. Theconverging section 18 of conduit 17 functions as an effuser causing thepressure of the sweep gas to decrease. This reduction in pressure causesthe partial pressure of permeate on the permeate side of structure 15 tobe lower than the partial pressure would be if the velocity of sweep gasin conduit 17 remained at constant velocity.

After passing semi-permeable structure 15, conduit 17 is passed througha diffuser 20 which functions as a compressor. As shown in FIG. 1,downstream of point 19 of conduit 17, the cross-sectional area of theconduit 17 diverges, thereby causing the velocity of the gas mixture ofpermeate and sweep gas to decrease and the pressure of the mixture toincrease. The diffuser 20 provides for the partial transformation of thekinetic energy of the gas mixture to an increased pressure.

FIG. 2 illustrates a sectional view of the fluid separation devicedepicted in FIG. 1 taken along lines 2-2. In this embodiment, conduits15 and 17 have a circular cross-section. However, other suitablecross-sectional designs of each conduit may be selected.

FIG. 3 illustrates a sectional, schematic view of a second embodiment ofthe present invention similar to FIG. 1 except that sweep conduit 27surrounds a semi-permeable structure 25 to provide supersonic velocityof the sweep gas across the semi-permeable structure 25. In theembodiment illustrated in FIG. 1, subsonic velocities are generated, butin some applications it may be desirable to generate sonic velocity inthe sweep gas close to the throat of an effuser so as to cause the sweepgas to expand supersonically as the sweep gas flows along the permeateside of the gas separation device.

Referring to FIG. 3, a multi-component, gas stream enters fluidseparation device 21 through flow conduit 24. Arrow 22 shows the flowdirection of the feed gas. A portion of conduit 24 has a semi-permeablestructure 25 through which one or more components of a multi-componentfluid stream selectively permeate therethrough. Arrows 26 show thedirection of permeate through conduit structure 25. A sweep fluid ispassed through flow conduit 27 in the direction of arrow 23. Thecross-sectional area of flow conduit 27 forms a supersonic effuser 35having the general shape of an axisymmetric nozzle comprising acylindrical section 28, a convergent truncated, subsonic, cone section29, a throat 30, and a divergent, supersonic section 31. The effuser 35is a de Laval-type of nozzle for inducing the sweep stream to flow atsupersonic velocity. The cross-sectional area of the sweep conduit 27that is concentric to the semi-permeable structure 25 is shown as beingconstant in FIG. 3, but optionally this portion of conduit 27 canprogressively increase or progressively decrease along the length of thesemi-permeable structure 25. The converging section 29 causes thepressure of the sweep gas to decrease. This reduction in pressure causesthe partial pressure of permeate on the permeate side of thesemi-permeable structure 25 to be lower than the partial pressure wouldbe if conduit 27 remained at constant velocity.

After passing semi-permeable structure 25, the gas mixture of permeateand sweep gas is passed through a diffuser 36 which comprises aconvergent, supersonic, truncated-cone section 37, a throat 38, and adivergent subsonic section 39. The diffuser 36 functions as acompressor, causing the velocity of the mixture of permeate and sweepgas to decrease and the pressure of the mixture to increase.

Depending on the application, any suitable rigid material can be usedfor the effuser 35 and diffuser 36.

Although not shown in FIG. 3, one or both of the effuser throat 30 anddiffuser throat 38 can be adjustable. The geometry and size of theeffuser 35 and diffuser 36 that ensure the desired velocity conditionsalong the permeate side of semi-permeable structure 25 can be chosen bythose skilled in the art on the basis of known laws of thermodynamics ofgas and the known initial data of the sweep gas flow, including forexample the sweep gas pressure at the entrance to the effuser 35, thetemperature of the sweep gas, and the chemical composition of the sweepgas.

The sweep gas used in the present invention can be any gas or vapor thatcontains a relatively low concentration of the one or more gascomponents to be removed from the multi-component feed gas. Nonlimitingexamples of a sweep gas may include hydrogen, air, steam, carbondioxide, carbon monoxide, and inert gases such as argon and helium.

The feed gas to the separation system 10 of FIG. 1 and the separationsystem 21 of FIG. 3 may derive from a variety of sources including, butnot limited to, industrial process vent streams, vaporous overhead froma distillation column, the overhead from a reflux process, chemicalprocess streams, and natural gas production from subterraneangas-bearing formations. The feed gas can comprise virtually anymulti-component gas mixture with sufficient volatility to be present inthe vapor phase.

The semi-permeable structure 25 for use in the present invention can beany suitable device having a selectively permeable nature and morespecifically it may be any device being relatively permeable to at leastone component relative to one or more other components in the feedstream. The semi-permeable structure 25 can be of any suitable designfor vapor separations. Tubular structures are preferred to obtain thebenefits of the partial pressure reduction on the permeate side of themembrane in accordance with this invention. The semi-permeable structure25 can be made entirely of the permselective material or thepermselective material may be supported on a porous structure, fabric,or screen. The semi-permeable structure 25 is preferably composed of aseparation layer and a support with the separation layer being formed onthe surface of the support. The support is designed to providemechanical support to the separation layer while offering as little masstransfer resistance as possible. The flux through the semi-permeablestructure is affected by the thickness of the separation material andthe support. In general it is desirable to have the separation layer,through which a permeating component must pass, as thin as possible yetsufficiently thick that the flow through the layer is not dominated bydefects. The support must be thick enough to provide adequate strengthto the separation layer to withstand the separation conditions. Suitablecomposite semi-permeable structure may comprise a thin separation layeror membrane formed on the surface of a thicker porous physical supportthat provides the necessary physical strength to the membrane. Thenumber and length of the individual membranes used in the semi-permeablestructure can be varied to suit the fluid flow rates and fluxrequirements of particular applications.

With respect to the composition of the separation layer, substantiallyany semi-permeable material currently available, or which may becomeavailable, can be used. The separation layer can be either symmetric orasymmetric, isotropic (having substantially the same density throughout)or anisotropic (having at least one zone of greater density than atleast one other zone), and can be chemically homogenous (constructed ofthe same material) or it may be a composite membrane.

When the membrane separation systems illustrated in FIGS. 1 and 3 areused to remove contaminants from natural gas stream, the separationlayer preferably is composed of material tolerant to temperatures above120° F. (48.9° C.) and pressures above 1,200 psia (82.8 bar) and haveadequate effective permeance and selectivity at those conditions. Maymembranes in service for acid gas removal from natural gas streams aremade from polymers, and most of these polymers either lack stability atthe operating conditions at temperatures above 120° F. (48.9° C.) andpressures above about 1,200 psia (82.8 bar) or do not provide adequatevalues of permeance or selectivity. Many of such polymeric membraneshave been designed or selected to operate most effectively attemperatures below about 100° F. (37.8° C.). While certain polymers orglassy materials could give adequate performance at higher temperatureand pressure conditions, it is preferred that the separation layer usedin natural gas treatment be inorganic. The inorganic layer, formed from,for example, zeolites, microporous silica, or microporous carbon, ispreferably placed on a structured support.

The support should offer minimal mass transfer resistance with strengthsufficient to withstand the stress created by relatively large pressuredifferentials across the membrane. For asymmetric membranes, the supportis porous. It is also possible to form an asymmetric hybrid membranestructure in which a polymeric active separation layer is coated onto aporous inorganic support. For asymmetric inorganic membranes, the poroussupport can be made from a different material than the active separationlayer. Support materials for asymmetric inorganic membranes includeporous aluminas, silicon carbides, porous metals, cordierties, andcarbons. Typically for asymmetric polymer membranes, the porous supportis manufactured from the some polymer as the active separation layer. Insome polymer membrane manufacturing processes, the porous supportmaterial is formed simultaneously with active separation layer.

The invention is not intended to be limited to any particular separationlayer or support, and the separation layer and support may comprise anymaterial capable of giving a values for permeance and selectivity. Thisincludes, for example, homogeneous membranes, composite membranes, andmembranes incorporating sorbents, carriers, or plasticizers. Inasmuch asthe composition and preparation of membrane are well known to thoseskilled in the art, a more detailed description thereof is not providedhe

FIGS. 1 and 3 illustrate embodiments in which the multi-component fluidto be treated and the sweep fluid are in countercurrent flow, which isthe preferred arrangement. However, co-current arrangements could alsobe produced, one embodiment of which is shown in FIG. 4.

FIG. 4 illustrates a sectional, schematic view of a third embodiment ofthe present invention showing an effuser 40 and diffuser 41 on theinside of a multi-component feed gas stream that is passing through aflow conduit 42 in the direction of arrow 43. A semi-permeable membranemodule 44 is disposed on the inside of a flow conduit 42. Membranemodule 44 comprises a membrane layer 45 that is coated or bonded to thesurface of a support member 46. The membrane module 44 may also includeother layers not shown FIG. 4, such as a protective layer that mayinclude for example a cage or screen to protect the outside membranelayer.

Sweep gas 49 enters the separation module through sweep gas inletconduit 48. The direction of sweep gas 49 into conduit 48. Inlet conduit48 passes through the bulb-shaped end of membrane module 44 and ends atnozzle 50, thereby enabling sweep gas 49 to flow through inlet conduit48 and exit through nozzle 50. The velocity of sweep gas 49 thoughnozzle 50 induces a low pressure zone in the throat 51 of a vent portion52 of the membrane module 44, drawing permeate through the membranelayer 45 to the interior of membrane module 44. A diffuser 41 is locateddownstream of the membrane module 44. The high velocity of the mixtureof permeate and sweep gas exiting the membrane module 44 is reduced invelocity in diffuser 41 to produce an increase in pressure over that ofthe gas mixture through section venturi portion 52.

In FIG. 4, diffuser 41 is shown as being positioned immediatelyfollowing the passage of the sweep gas past membrane module 44. However,the diffuser 41 may optionally be positioned father downstream thanshown in FIG. 4. The diffuser 41 may optionally be outside conduit 42.

The method of the present invention may be practiced in any flowenvironment involving two or more concentric flow lines in which atleast a portion of the inner conduit has a semi-permeable wall forselective permeation of one or more components of a multi-component gasstream. The concentric flow lines may be pipelines located above orbelow the surface of the earth or the flow lines may comprise componentsof a wellbore, such as a tubing string and casing in a well thatproduces natural gas from one or more subterranean formations.

A person skilled in the art, particularly one having the benefit of theteachings of this patent, will recognize many modifications andvariations to the specific processes disclosed above. For example, avariety of temperatures and pressures may be used in accordance with theinvention, depending on the overall design of the system and thecomposition of the feed gas. Also, the feed gas cooling train may besupplemented or reconfigured depending on the overall designrequirements to achieve optimum and efficient heat exchangerequirements. As discussed above, the specifically disclosed embodimentsshould not be used to limit or restrict the scope of the invention,which is to be determined by the claims below and their equivalents

1. A method of separating a component from a multi-component gas,comprising: (a) providing a flow conduit having a semi-permeable sectionadapted to selectively permeate the component to be separated in thepresence of the multi-component gas flowing therethrough, the flowconduit having a feed side and a permeate side; (b) passing themulti-component gas along the feed side of the flow conduit; (c)providing a sweep gas at a first velocity, the sweep gas being suitablefor passage along the permeate side of the flow conduit and beingsuitable for sweeping the component gas that permeates through thepermeable section of the conduit away from the permeate side of the flowconduit, thereby producing a gas mixture comprising the sweep gas andthe component gas; (d) accelerating the velocity of the sweep gas sothat the velocity of the sweep gas along at least a portion of thepermeate side of the flow conduit is greater than the first velocity ofthe sweep gas; and (e) decelerating the gas mixture by means of adefuser, thereby recovering as pressure a portion of the energy of thegas mixture.
 2. The method of claim 1 wherein the multi-component gas isnatural gas.
 3. The method of claim 1 wherein the sweep gas isaccelerated to supersonic velocity.
 4. The method of claim 1 wherein thestep (e) of decelerating the gas mixture is carried out by passing thegas mixture through a supersonic diffuser.
 5. The method of claim 1wherein the multi-component gas is rich in methane and contains CO₂ andthe CO₂ is the component being permeated through the semi-permeablesection, and the sweep gas is lean in CO₂.
 6. A membrane separationsystem for separating one or more components from a multi-component gas,comprising: (a) a first flow conduit longitudinally positioned inside aportion of a second flow conduit, the first conduit adapted for flow ofthe multi-component gas therethrough, the first flow conduit having asemi-permeable membrane for permeation therethrough of one or morecomponents of the multi-component gas; (b) the second conduit adaptedfor passage of a sweep fluid to facilitate removal of permeate on thepermeate side of the membrane, the second conduit having a first areafollowed by a smaller second flow area, the second flow area beingconcentric to at least a portion of the membrane; and (c) the secondconduit having a third flow area in a downstream direction of the secondflow area, the third flow area being greater than the second flow area.7. The membrane separation system of claim 6 wherein the second flowarea continuously decreases concentrically along a substantial portionof membrane portion of the first conduit.
 8. The membrane separationsystem of claim 6 wherein the second flow area decreases to-a fourthflow area and then increases to a fifth flow area, the fifth flow areabeing concentric over a substantial portion of the membrane portion ofthe first conduit.
 9. The membrane separation system of claim 8 whereinthe fifth flow area decreases in area to a sixth flow area in thedownstream direction of the second conduit, the sixth flow area beingsubstantially the same at the fourth flow area.